An image forming device always has a toner cartridge for supplying a toner to a developing device. The toner cartridge is detachably mounted on the image forming device, and when the toner in the toner cartridge is used up, the user can replace it by himself. In order to prevent a toner container from not being mounted at the right position in the image forming device (i.e., the so-called half-inserted state), an engagement mechanism is provided on the side of the body of the image forming device, for the engagement of the toner cartridge. However, the above mentioned mechanism for preventing half-insertion of the toner container has a defect that, the engagement mechanism must be released one by one when it is necessary to remove the toner cartridge from the body of the image forming device.